1. Field of the Invention
The invention relates generally to fixed-head drill bits, and in particular to fixed-head drill bits having stabilizing features for, inter alia, improving stability while drilling.
2. Background Art
FIG. 1 shows a conventional fixed-head drill bit 5, sometimes referred to as a “fixed cutter” drill bit, for drilling into subterranean formations. Fixed-head bits typically rotate as one piece and contain no separately moving parts. Bit 5 typically includes a bit body 10 having an externally threaded connection for connecting to a drill string at one end 12, and a plurality of blades 14 extending from the other end of bit body 10. A plurality of cutting elements 16, sometimes referred to as “fixed cutters,” each defining a cutting surface, are attached to the blades 14 to cut through earth formations when the bit 5 is rotated during drilling. The cutting elements 16 deform the earth formation by scraping and shearing. The cutting elements 16 may be tungsten carbide inserts, polycrystalline diamond compacts, milled steel teeth, or any other cutting elements of materials hard and strong enough to deform or cut through the formation. Hardfacing (not shown) may also be applied to the cutting elements 16 and other portions of the bit 5 to reduce wear and increase the life of the bit 5.
Polycrystalline diamond cutting elements are frequently used on fixed-head drill bits. One embodiment of polycrystalline diamond includes polycrystalline diamond compact (“PDC”), which comprises man-made diamonds aggregated into relatively large, inter-grown masses of randomly oriented crystals. Polycrystalline diamond is highly desirable, in part due to its relatively high degrees of hardness and wear resistance. Despite these properties, however, polycrystalline diamond will eventually wear down or otherwise fail after continued exposure to the stresses of drilling. Undesirable bit performance such as vibration and whirling while drilling exacerbates wear and tear on the cutting elements.
Many approaches have been devised to improve drill bit dynamic characteristics to reduce the detrimental effects to the drill bit. In particular, stabilizing features known as “wear knuckles”, sometimes interchangeably referred to as “contact pads” or “wear knots”, are used to stabilize the drill bit by controlling lateral movement of the bit, lateral vibration, and depth of cut. These stabilizing features project from the bit face, either trailing or leading a corresponding cutting element with respect to a rotational direction about a bit axis.
U.S. Pat. No. 6,568,492 discloses an example of a combination mill/drill bit employing stabilizing features referred to as “secondary ridge structures.” The bit has primary cutting elements and secondary structures intended to enable continuous substantially smooth milling of down hole casing and subsequent drilling of an earth formation. The primary cutting elements are inserts made of polycrystalline diamond or other hard material. Secondary ridge structures having relatively blunt protrusions are intended to protect the primary cutting elements by absorbing impacts, limiting the primary cutting element engagement, controlling torque, and providing stability.
U.S. Pat. No. 6,659,199 discloses a rotary bit design including stabilizing features referred to as “elongated bearings.” The elongated bearings are designed to travel within a tubular clearance volume defined by the path of a respective cutting element drilling through the formation. This placement of the bearing requires anticipating the helical path cut by the cutting element, which is a function of parameters such as: rates of penetration and rotational speeds. This placement is intended to minimize contact between the elongated bearing and the uncut rock adjacent the helical path cut by the cutting element.
One characteristic of fixed-head bits having conventional stabilizing features is that the cutting elements extend outwardly of the stabilizing features, to contact the formation in advance of the stabilizing features. The stabilizing features are designed not to contact the formation until the bit advances at a selected minimum rate or depth of cut (“DOC”). In many cases, stabilizing features therefore do not sufficiently support the fragile cutting surface. In other cases, the cutting elements may penetrate further into the formation than predicted by the stabilizing features, so that the cutting tips become overloaded despite the presence of the stabilizing features. Furthermore, the manufacturing process used to create these bits may not allow the accuracy required to consistently reproduce a desired minimum DOC. One or more stabilizing features may contact the formation while others have clearance. This imbalance can introduce additional instability. Therefore, an improved apparatus and method for stabilizing a drill bit are desirable.